wp_nav_menu() gives different results when called multiple times - reproducable

Description

If you call wp_nav_menu() with the same parameters twice, you get different results.

To reproduce
1) use the TwentyTen theme
2) add two categories: CAT-PARENT and CAT-CHILD
3) add two pages: PAGE-PARENT and PAGE-CHILD
4) add a menu, add the two categories and two pages and adjust them so the children are under the parents
5) edit the header.php from twentyten and after line 67 (which should be
' </div><!-- #branding -->'
add
<?php
echo '<strong>wp_nav_menu=<br></strong>';
wp_nav_menu( array( 'container_class' => 'menu-xxx', 'theme_location' => 'primary' ) );

echo '<strong>END wp_nav_menu<br></strong>';

?>
<?php
echo '<strong>wp_nav_menu=<br></strong>';
wp_nav_menu( array( 'container_class' => 'menu-xxx', 'theme_location' => 'primary' ) );
echo '<strong>END wp_nav_menu<br></strong>';
?>
6) view the site and the generated source
This is the output of the two calls to wp_nav_menu():

notice that the <li> in the first call have an id="menu-item-nnnn" and the second call doesn't have that. I know that an ID can only happen once in a page, but since there is a class with the same value, it would seem that neithor call should have an id="..." returned to be consistant.